Sintering furnace

ABSTRACT

A tube furnace for sintering parts which are loaded with organic products, the internal atmosphere of said furnace being heated to a high temperature by means of an electric resistor. The furnace comprises a tube surrounded by at least one layer of heatinsulating refractory material and an outer metallic casing which is adapted to cooperate at the extremities with leaktight doors. The refractory layer is traversed by a system of pipes through which the sintering gases flow and which serve to heat these latter in the vicinity of the heating resistor prior to injection of the gases at both ends of the furnace tube. A transverse slot is formed at the center of the tube and communicates with a chimney which traverses the refractory layer and outer casing. The chimney is fitted with a system whereby the carbonaceous compounds resulting from the decomposition of organic products during the sintering process are trapped by condensation.

United States Patent Inventors Yves J. M. Masselot Manosque; Georges Pouiol, Aix-en-Provence, both 0!, France Appl. No. 860,186 Filed Sept. 23, 1969 Patented Sept. 7, 1971 Assignee Commlseeriot L'Energie Atomiqne Paris, France Priority Sept. 27, 1968 France SINTERING FURNACE 3 Claims, 1 Drawing 11;.

11.8. CI. 13/20, 75/224 Int. CL 1105b 3/62 -Field of Search 75/5, 224;

56 References Cited UNITED STATES PATENTS 2,203,895 6/1940 Davis et al. 75/224 3,403,212 9/1968 Sennosuue Sato 13/22 X Primary Examiner-Hemard A. Gilheany Assistant Examiner -Roy N.. Envall, Jr. Attorney-Cameron, Kerkam & Sutton ABSTRACT: A tube furnace for sintering parts which are loaded with organic products, the internal atmosphere of said furnace being heated to a high temperature by means of an electric resistor. The furnace comprises a tube surrounded by at least one layer of heat-insulating refractory material and an outer metallic casing which is adapted to cooperate at the extremities with leaktight doors. The refractory layer is traversed by a system of pipes through which the sintering gases flow and which serve to heat these latter in the vicinity. of the heating resistor prior to injection of the gases at both ends of the furnace tube. A transverse slot is fonned at the center of the tube and communicates with a chimney which traverses the refractory layer and outer casing. The chimney is fitted with a system whereby the carbonaceous compounds resulting from the decomposition of organic products during the sintering process are trapped by condensation.

PATENTEU SEP 1 am j ATTORNEY SINTERING FURNACE This invention relates to an industrial sintering furnace which is primarily although not exclusively intended for baking in a reducing or inert gaseous atmosphere radioactive metallic or refractory pellets and more especially pellets formed of the oxides of fissile materials such as uranium and plutonium.

It is known that the fabrication of pellets of this type by sintering consists first in agglomerating the powder of the material considered with a suitable binder and compacting the product obtained, then in carrying out within a furnace which is usually of the tube type a high-temperature backing process in a reducing atmosphere during which in particular the binder employed is eliminated. In the case in which said binder is loaded with organic compounds, a number of disadvantages immediately appear and make it difficult to employ a conventional sintering furnace. In the first place, the gas flow usually proves to be insufficient for the purpose of carrying away to the exterior the products of cracking of the organic materials. Moreover, the circulation of gases along the axis of the tubular furnace from one end to the other has the effect of assisting condensation of coal tar products and deposition of lampblack on the doors which close the furnace, these doors being usually'cooled by a circulation of water in order to protect the seals from the action of heat and to stop any thermal leakage towards the exterior of the furnace. The deposits referred to can have the effect of short circuiting electric connections to the heating resistors and of clogging the openings through which the residual gases are discharged.

Finally, in conventional furnaces, the condensations and deposits referred to above result in darkening of the gas stream and consequently prevent determination of temperature within the furnace by optical viewing means while sintering operations are in progress. This in turn results in loss of operating time by reason of the cleaning operations which have to be carried out periodically.

The present invention is intended to provide a sintering furnace which overcomes the disadvantages mentioned above by permitting on the one hand an increase in the rate at which the gas flows through the furnace and carries away the carbonaceous products of decomposition without thereby resulting in cooling which is detrimental to sintering of parts and by making it possible on the other hand as a result of arrangement of the circulation of gases especially from each end towards the center of the furnace to prevent condensation and formation of deposits on the furnace doors, the entrained products being collected in the hottest zone and then readily removed.

To this end, the sintering furnace under consideration comprises an axial cylindrical tube for accommodating the parts to be sintered which are loaded with organic products, the internal atmosphere of said furnace being heated to a high ternperature by means of an electric resistor. The furnace tube is surrounded by at least one layer of heat-insulating refractory material and said layer is in turn enclosed in an outer metallic casing which is adapted to cooperate at the extremities with leaktight closure doors. The furnace is characterized in that said refractory layer is traversed by a system of pipes through which the sintering gases flow and which serve to heat said gases in the vicinity of said electric resistor prior to injection into the furnace tube at both extremities thereof, said tube being provided in the central portion thereof with a transverse slot which is adapted to communicate with a chimney formed through said layer and said casing, said chimney being fitted with a system whereby the carbonaceous compounds resulting from the decomposition of organic products during the sintering process are trapped by condensation.

As an advantageous feature, said trapping system comprises a nest of metallic hairpin tubes within which a cooling fluid is circulated, said hairpin tubes being rigidly fixed to a series of transverse plates arranged in staggered relation and supported by a flange plate for closing said chimney, said flange plate being provided with an axial orifice for the discharge of gases which is connected to external filters by means of a demountable pipe.

Further properties of a sintering furnace in accordance with the invention will appear from the following description of one example of construction which is given by way of indication and not in any limiting sense, reference being made to the single FIGURE of the accompanying drawings which illustrate a longitudinal sectional view of the furnace under consideration.

As shown in this FIGURE, the sintering furnace which is illustrated mainly comprises a cylindrical tube 1 disposed in the central portion of the furnace and intended to accommodate parts to be sintered (not shown in the drawings), and especially oxide pellets of fissile material. The tube 1 is surrounded externally by a suitable thickness of a heat-insulating refractory material constituted in the exemplified embodiment herein described by two concentric layers each formed by an assembly of small bricks 2 which are suitably disposed in contact with each other. The layers of bricks 2 are surrounded externally by a metallic casing 3 which is closed at both ends by means of doors 4 and 5, said doors being intended to be applied against flanges formed on said casing and in oppositely facing relation with interposition of seals 6 and 7. As an advantageous feature, the doors 4 and 5 are continuously cooled by either water or any other suitable coolant which is circulated in continuous flow.

Heating of the tube 1 is carried out by means of a ribbon resistor 8 which is formed especially of molybdenum and folded in the form of a snaked coil so as to surround the outer surface of the tube, said ribbon resistor being preferably mounted inside recesses which are advantageously formed in the corresponding faces of the first layer of refractory bricks 2. The extremities of the resistor 8 are connected by means of leadwires 9 and 10 to terminals 11 and 12 and these latter are in turn connected to a current supply point 13 which is placed on the metallic casing 3.

In accordance with the invention, the furnace under consideration comprises a system for supplying the tube 1 with reducing gases which are necessary for the sintering operation proper. Said system consists of a series of pipes 14 which pass through the first layer of refractory bricks 2 in the vicinity of the ribbon resistor 8, thereby directly heating these gases as they pass through the pipes 14. The gas is admitted into said pipes in the central portion of the furnace by means of a duct 15 which traverses the metallic casing 3 and opens into an annular header 15a which is formed in the stack of refractory bricks and serves to distribute the gases on each side through the extremities 16 of the pipes 14 in the direction of the edges of the tube 1, the pipes being directly connected to the header. Under these conditions, the gases pass through only a halflength of the furnace prior to being admitted into the tube 1 through the extremities of this latter. It should be noted that this arrangement in which provision is made for the use of a single feed header avoids multiple penetrations through the furnace doors 4 and 5.

In accordance with another important feature of the invention, a transverse slot 17 is formed in the central zone of the tube I. Said slot extends over the major part of the contour of the tube and opens into a chimney 18 which is formed through the layers of refractory bricks 2 and the outer metallic casing 3. Said chimney 18 is limited by a cylindrical sleeve 19 which is joined to the casing 3 and fitted in the vicinity of the tube 1 with refractory elements 20 which maintain the gases at a high temperature as they are discharged from the tube 1 through the slot 17. There is mounted coaxially with the sleeve 19 a trapping system 21 constituted by a nest of small hairpin tubes 22 through which a flow of water in particular is circulated and by transverse metallic plates 23 which are welded to the hairpin tubes 22. Said plates 23 are advantageously mounted parallel to each other and disposed in such a manner as to cause the gases to flow in a staggered path in the direction of the arrows 24. The hairpin tubes 22 are supported at the upper extremities thereof by a closure plate 25 which seals off the chimney l8. Said plate has a central aperture 26 which is joined to a detachable pipe 27 for discharging the gases to external filters (not shown in the drawings) which are provided for retaining radioactive dust particles.

By virtue of these arrangements, the carbonaceous products of the decomposition resulting in particular from the process of cracking of organic compounds which constitute the binder of the sintered parts within the tube 1 are carried by the flow of gas through the pipes 14 from the ends of the tube towards the central zone of this latter, then pass out directly through the slot 17 towards the trap 21. Said products which are maintained in the gaseous state as a result of the high temperature within the tube then follow a staggered path through the plates 23 and come into contact with the outer surface of the hairpin tubes 22 on which they condense, thereby preventing any formation of deposits within the furnace proper and especially on the closure doors 4 and which are maintained at a lower temperature. The residual gases which are thus freed from their tar products, lampblack, soot and so forth are then discharged to the exterior of the furnace through the pipe 27. Disassembly of the trap 21 can be carried out if necessary simply by removing the closure plate 25, the hairpin tubes 22 and the plates 23 being readily brushed and cleaned without requiring any further maintenance work on the furnace itself. Provision can advantageously be made for a system of molybdenum baffles (not shown) for the purpose of protecting the extremity of the nest of hairpin tubes from radiation emanating from the interior of the furnace tube 1.

The purification of the furnace atmosphere which is thus achieved by means of the trapping system has one notable result in that it facilitates optical sighting on temperature-measuring pyrometers which is usually carried out through quartz windows in the end doors of the furnace. Moreover, the circulation of the gases from the ends of the central tube towards the central portion of this latter prevents any deposition on the doors and thus eliminates any risk of a short circuit across the terminals which provide an electric connection with the heating resistor Finally, the furnace in accordance with the invention makes it possible to increase the rate of flow of the gases within the central tube to an appreciable extent without thereby lowering the temperature within the furnace since said gases are subjected on their flow path to a preliminary heating prior to introduction into the tube in the direction of the sintering z one.

There is thus provided ah ndustrial furnace of very simple design in which the functions of sintering on the one hand and trapping of impurities which are carried away by the gases on the other hand are strictly separated from each other. The operations involving cleaning of the trap can therefore be carried out wholly independently of either positioning or withdrawal of parts to be sintered. A set of two standard traps makes it possible to place a clean trap in position during operations involving charging and discharging of pellets, thereby avoiding any lengthening of the sintering cycle.

It will be readily apparent that the invention is not limited in any sense to the exemplified embodiment which has been more especially described with reference to the accompanying drawings and extends on the contrary to all alternative forms.

What we claim is:

l. A sintering furnace comprising an axial cylindrical tube for accommodating the parts to be sintered which are loaded with organic products and the internal atmosphere of which is heated to a high temperature by means of an electric resistor, said tube being surrounded by at least one layer of heat-insulating refractory material which is in turn enclosed in an outer metallic casing adapted to cooperate at the extremities thereof with leaktight closure doors, characterized in that said refractory layer is traversed by a system of pipes through which the sintering gases fiow and which serve to heat said gases in the vicinity of said electric resistor prior to injection into said tube at both extremities thereof, said tube being provided in the central portion thereof with a transverse slot which is adapted to communicate with a chimney formed through said layer and said casing, said chimney being fitted with a system whereby the carbonaceous compounds resulting from the decomposition of organic products during the sintering process are trapped by condensation.

2. A sintering furnace in accordance with claim 1, characterized in that said trapping system comprises a nest of metallic hairpin tubes within which a cooling fluid is circulated, said hairpin tubes being rigidly fixed to a series of transverse plates arranged in staggered relation and supported by a flange plate for closing said chimney, said flange plate being provided with an axial orifice for the discharge of gases which is connected to external filters by means of a detachable pipe.

3. A sintering furnace in accordance with claim 1, characterized in that said heating pipes are supplied with the sintering gases by means of an annular header which is mounted in the central portion of said furnace within said layer of heat-insulating refractory material. 

1. A sintering furnace comprising an axial cylindrical tube for accommodating the parts to be sintered which are loaded with organic products and the internal atmosphere of which is heated to a high temperature by means of an electric resistor, said tube being surrounded by at least one layer of heat-insulating refractory material which is in turn enclosed in an outer metallic casing adapted to cooperate at the extremities thereof with leaktight closure doors, characterized in that said refractory layer is traversed by a system of pipes through which the sintering gases flow and which serve to heat said gases in the vicinity of said electric resistor prior to injection into said tube at both extremities thereof, said tube being provided in the central portion thereof with a transverse slot which is adapted to communicate with a chimney formed through said layer and said casing, said chimney being fitted with a system whereby the carbonaceous compounds resulting from the decomposition of organic products during the sintering process are trapped by condensation.
 2. A sintering furnace in accordance with claim 1, characterized in that said trapping system comprises a nest of metallic hairpin tubes within which a cooling fluid is circulated, said hairpin tubes being rigidly fixed to a series of transverse plates arranged in staggered relation and supported by a flange plate for closing said chimney, said flange plate being provided with an axial orifice for the discharge of gases which is connected to external filters by means of a detachable pipe.
 3. A sintering furnace in accordance with claim 1, characterized in that said heating pipes are supplied with the sintering gases by means of an annular header which is mounted in the central portion of said furnace within said layer of heat-insulating refractory material. 